This invention relates generally to packing materials and, in particular, to a packing material that is useful for sealing mechanical pumps.
Packing materials of the type described herein are used mechanical pumps at junctions where a rotary shaft or reciprocating rod extends into a fluid-filled cavity. The fluid-filled cavity can be filled with any of a wide range of chemical fluids or solvents, many of which are highly corrosive. It is important that the packing material be resistant to these corrosive chemicals.
In addition to chemical resistance, suitable packing materials should withstand fluid temperatures or heat generated by mechanical friction of up to about 500.degree. F., while functioning to assist in the dissipation of heat. The packing material must exhibit minimal thermal dimensional change so that the fluid filled cavity remains sealed in such a way as to prevent leakage. Accordingly, the packing materials should be sufficiently resilient to retain their original shape under thermal, chemical and mechanical actions.
Packing materials for these purposes are known. Until several years ago, asbestos was a standard widely used material for braided "compression" packings. However, asbestos use has fallen into disfavor since its carcinogenic properties have become known. Various packing materials have been proposed and several are satisfactory for some applications.
For example, several utilize glass or an inorganic fiber as proposed in U.S. Pat. No. 3,306,155 issued to Zumeta and Case on Feb. 28, 1967 who disclose a braided packing material having glass braids impregnated with polytetrafluoroethylene resin. The braided structure can be enclosed in a polytetrafluoroethylene jacket.
Similarly, Zumeta and Case in U.S. Pat. No. 3,534,652 issued on Oct. 20, 1970 disclose a high temperature packing material formed of a braided core of glass or silicon dioxide filaments impregnated with molybdenum disulfide. The core can be provided with a surface coating of molybdenum disuifide and/or a braided graphite jacket.
U.S. Pat. No. 4,371,180 to Case and Hopper on Feb. 1, 1983 discloses a gasket material that combines a braided organic fiber with a vitreous fiber sleeve. The gasket material is impregnated with polytetrafluoroethylene as a sealant. In another embodiment, a sleeve is braided of yarn containing both inorganic and organic fibers that are either spun or twisted together. The use of vitreous fibers makes it possible to use the gasket material to retain fluid at pressures higher than are possible with organic fiber alone.
U.S. Pat. No. 4,298,207 issued to Hopper et al on Nov. 3, 1981 discloses a gasket material having a resilient core of glass fiber and a jacket of glass or other suitable fiber over the core. An essentially impermeable sheath of a chemically resistant material can be provided between the jacket and the core. Both the jacket and core can be impregnated with polytetrafluoroethylene.
U.S. Pat. No. 4,431,698 issued to Case et al on Feb. 14, 1984 discloses a low friction, water and steam resistant packing containing an inorganic fiber in combination with dispersed and dried, but unsintered, polytetrafluoroethylene. The inorganic fiber can be any of chemical, electrical, structural and alkali resistant glasses, ceramic, quartz, carbon and graphite materials. The packing is prepared by combining the inorganic fiber with a polytetrafluoroethylene dispersion containing a wetting agent and heating the resulting combination to a temperature high enough to decompose the wetting agent but insufficient to sinter the particles of the dispersion. Accordingly, a packing that is essentially free of a wetting agent is prepared.
U.S. Pat. No. 4,559,862 issued to Case et al on Dec. 24, 1985 discloses a packing material comprising glass or other inorganic fibers in combination with an organic fiber such as sintered polytetrafluoroethylene. The packing material can be used either with or without an impregnant. Preferred impregnants include lubricants with a binder.
All organic packings in U.S. Pat. No. 3,791,658 to Zumeta and Case on Feb. 12, 1974 have an inner Teflon-containing core and an outer carbon-containing jacket surrounding the core. The core can be Teflon fiber or Teflon impregnated packing material such as asbestos or fiberglass. The outer jacket can be a graphite filament that is either wrapped or braided around the core. Similarly, Houghton et al in U.S. Pat. No. 3,646,846 which issued on Mar. 7, 1972 disclose a packing material comprising graphite strands with interstices between the strands that are thoroughly impregnated with an impregnating material of a fluorocarbon resin and a solid lubricant such as graphite.
These prior art packing materials are suitable for some applications, but are not completely satisfactory, particularly in high temperature highly corrosive applications. Accordingly, it is desirable to provide an improved packing material having superior properties.